By comparison, presumptive Democratic nominee Hillary Clinton, who had yet to clinch her party's nomination, raised $28 million in May through the campaign and its "victory fund," finishing the month with $42.5 million on hand.

Cuban, who has flirted with the idea of serving as either Trump's or presumptive Democratic nominee Hillary Clinton's running mate, has trolled Trump over his perceived wealth recently.

The billionaire business mogul, star of ABC's "Shark Tank," and owner of the NBA's Dallas Mavericks said earlier this month that "it's not even close" as to who has more in liquid assets between the two. He also questioned whether Trump was a billionaire.